Instrument for measuring plasma excited by high-frequency

ABSTRACT

This instrument can measure parameters of a plasma accurately and easily even though the plasma is exited by a high-frequency. The instrument for measuring parameters of a plasma generated in a vacuum chamber by high-frequency discharge at a given frequency comprises a wire (106) for electrically connecting a first electrode (101) arranged in a space where a plasma is produced and a terminal (110) arranged outside the vacuum chamber for taking out signals, and a first insulator (105) so arranged as to cover at least a part of the surface of the wire therewith. The absolute value of the impedance at the given frequency between the first electrode and the ground when looking into the terminal side from the first electrode is five times or more the absolute value of the impedance at the given frequency between the first electrode and the plasma in a state where no direct current flows through the first electrode.

TECHNICAL FIELD

The present invention relates to an instrument for measuring a plasmaexcited by a high frequency, and relates to an instrument which makespossible the accurate measurement of all parameters of a plasma excitedby a high frequency.

BACKGROUND ART

By applying a voltage to an extremely small electrode inserted into astationary plasma produced by means of direct current discharge, andthus measuring the amount of current flowing from the plasma, it ispossible to measure plasma parameters such as the potential, density,electron temperature and the like of the plasma; beginning with thesingle probe proposed in the 1920's by Langmuir et al, various probeshave been employed in plasma measurement, such as the emission probe,which was proposed later and which employed thermoelectrodes, the doubleprobe, which employed two electrodes, and the like. These probes wereeffective in the measurement of direct current discharge plasma in whichthe plasma potential did not fluctuate over time; however, when used inthe measurement of high frequency discharge plasma, such probes weregreatly affected by the excitation frequency of the plasma, and thiscaused a problem in that the accuracy of measurement of the plasmaparameters declined markedly. In addition, it was difficult to obtainaccurate information about the film formation atmosphere of, forexample, functional thin films, or the like, and this constituted agreat hindrance to the development of high quality functional thinfilms.

The present invention has as an object thereof to provide an instrumentfor measuring plasma which is capable of accurately and simply measuringall parameters of plasma, not only in direct current discharge plasma,but also in high frequency discharge plasma.

DISCLOSURE OF THE INVENTION

The measuring instrument for plasma excited by high frequencies inaccordance with the present invention is a measuring instrument formeasuring all values of a plasma produced within a vacuum chamber bymeans of a high frequency discharge having a given frequency,characterized in comprising a wire for electrically connecting a firstelectrode disposed within a plasma and a terminal provided outside thevacuum chamber which serves to take out signals, and a first insulatorwhich is provided so as to cover at least a portion of the surface ofthe wire, and in that the absolute value of the impedance at the givenfrequency between the first electrode and the ground when looking intothe terminal side from the first electrode is 5 times or more theabsolute value of the impedance at the given frequency between the firstelectrode and the plasma in a state in which no direct current flowsthrough the first electrode.

It is preferable that a second electrode be disposed within the plasma,and that the second electrode and the first electrode be connected via acapacitor.

Furthermore, it is preferable that the impedance between the terminaland the ground when looking into the side opposite the first electrodefrom the terminal, at the given frequency, be made variable.

Furthermore, it is preferable that at least a portion of the wirecomprises a coaxial cable, that the length of the outer conductor ofthis coaxial cable be approximately equivalent to an odd multiple of 1/4the wavelength within the coaxial cable at the given frequency, and thatthe core and the outer conductor of the coaxial cable be provided via acapacitor at the terminal side end of the coaxial cable.

Additionally, it is preferable that at least a portion of the wirecomprise a coaxial cable, that a resistor be provided between the firstelectrode and the core of the coaxial cable, and that a measuringinstrument which is capable of measuring the high frequency voltage ofthe terminal at the given frequency be provided.

Function

Hereinbelow, the function of the present invention will be explained onthe basis of experiments which were conducted in the process of arrivingat the present invention.

FIG. 12 shows the results of the measurement of a direct currentdischarge plasma and a high frequency discharge plasma using aconventional single probe. The horizontal axis indicates the voltageapplied to the probe electrode, the left hand vertical axis indicatesthe current value of the probe electrode, and the right hand verticalaxis indicates the electron current value. The solid and dotted linesindicate the results of the measurement with respect to, respectively,the direct current discharge plasma and the high frequency dischargeplasma. Here, by measuring the ion energy present in the plasma using anion energy analyzer, the discharge conditions were set so that theplasma potential of both the direct current discharge plasma and thehigh frequency discharge plasma were identical, and the plasma wasproduced.

It was learned that, as shown in FIG. 12, the current/voltagecharacteristics of the direct current discharge plasma and the highfrequency discharge plasma differed completely. In the direct currentdischarge plasma, the plasma potential, for example, as obtained fromthe current voltage characteristics agreed well with the value obtainedfrom the ion energy. In contrast, in the high frequency dischargeplasma, the average plasma potential value was 34 V lower than theactual value, and the observed plasma density was only 1/3 of thatexpected.

This is thought to occur for the following reasons. In the highfrequency discharge plasma, the plasma potential fluctuates greatly withthe excitation frequency. The potential of a probe electrode which isinserted into such a plasma also fluctuates with the excitationfrequency; however, the amplitude and phase of the potential of theelectrode was different in conventional probes from the amplitude andphase of the plasma potential. The potential of the probe electrode wasdetermined by the impedance of a sheath provided around the electrodeand the impedance of the probe itself, and the amount of current flowingin the probe electrode depended on the difference between the plasmapotential and the potential of the probe electrode, so that the currentvalue of the probe also varied with the excitation frequency. As aresult, the dependency of the average value of the probe current withrespect to the potential of the probe electrode differed completely fromthat in the case of a direct current discharge plasma, as shown in FIG.12, and it was thus difficult to accurately extract the parameters suchas the plasma potential, density, electron temperature, and the like.Accordingly, it was determined that in order to obtain current voltagecharacteristics identical to those of the direct current dischargeplasma, the probe electrode potential should be made to oscillate insuch a manner as to be parallel to and synchronous with the plasmapotential. It was determined that for this reason, it was necessary toset the impedance of the probe so as to be sufficiently larger than theimpedance of the sheath. Next, the relationship between the probeimpedance and the sheath impedance will be explained.

FIG. 13 shows how the measured values of the plasma potential (solidline) and electron density (dotted line) vary with respect to thesusceptance of the probe itself. The probe which was used in themeasurements had a variable susceptance at the excitation frequency, andthe loss thereof was sufficiently small, so that the resistancecomponent of the electrode could be ignored. The actual plasma potentialand electron density were equivalent to the measured values in the casein which the susceptance was 0, that is to say, the impedance (inactuality, the reactance) was infinitely large. It can be seen from FIG.13 that whether the probe is capacitive (the susceptance is positive) orinductive (the susceptance is negative) the accurate measurement of theplasma parameters is possible only in the region in which thesusceptance is small. The value which varies the most with respect tovariations in the susceptance is the measured value of the electrondensity; however, in order to measure the actual electron density withan accuracy of within 10% so as to, for example, stably produce highfunction thin films with good reproducibility, it is necessary to setthe absolute value of the susceptance to a level of approximately 0.2 mSor less.

FIG. 14 shows how the measured values of the plasma density vary withrespect to the resistance value of the probe. The probe which was usedfor the measurement had a variable resistance, and it was possible toignore the reactance component thereof. The conditions of the plasmameasured were identical to those in FIG. 13. It can be seen from FIG. 14that in order to measure the actual plasma density to within an accuracyof 10% or less, the resistance value of the probe must be 5 kΩ or more.This is in agreement with the case shown in FIG. 13 in that the absolutevalue of the impedance of the probe must be 5 kΩ or more. That is tosay, a susceptance of 0.2 mS corresponds to an impedance of 5 kΩ.Furthermore, when both the resistance component and the reactancecomponent were made variable, it was confirmed that if the absolutevalue of the impedance of the probe was 5 kΩ or more, essentiallycorrect measurement was possible.

The capacitance of the sheath between the probe electrode and the plasmavaries with the voltage applied to the sheath, the plasma density, theshape of the probe electrode, and the like. In the cases shown in FIGS.13 and 14, the capacitance of the sheath when the probe was madefloating in a direct current manner was 1.55 pF. In FIGS. 13 and 14, 100MHz was employed as the excitation frequency of the plasma; at thisfrequency, 1.55 pF resulted in 1.03 kΩ. Accordingly, in order to conductan accurate measurement within 10% it is necessary to set the absolutevalue of the impedance of the probe itself to 5 kΩ or more, and inconsideration of this, the impedance of the probe itself should be atleast 5 times the impedance of the sheath capacitance in the case inwhich the probe is made floating in a direct current manner.

On the other hand, in a conventional single probe, as shown in, forexample, FIG. 15, an electrode portion 1501 is connected to a DC powersource and an ammeter via a core, the core is shielded with a metal pipe1503, and the metal pipe is grounded. Accordingly, the capacitancebetween the probe electrode and the grounding point is normally 100 pFor more, and thus the impedance is capacitive and small. It wasdetermined with respect to such probes that in order to conduct theaccurate measurement to within 10% which was described above, themaximum allowable capacitance was roughly 0.3 pF, and thus the accuratemeasurement of plasma was completely impossible with probes of aconventional structure. For the same reasons, standard double probes,emission probes, and the like were also unsuitable for the measurementof high frequency discharge plasma.

With respect to double probes, a structure was considered for themeasurement of high frequency excitation plasma in which a low passfilter was employed so that the two probes were made floating withrespect to the ground at the excitation frequency of the plasma;however, in order to reduce the parasitic capacitance between the probeand the ground, it is necessary to isolate the probe and the chamberwalls, and the appropriate chamber structures are limited. Furthermore,the production of that portion of the probe which extends outside of thechamber is extremely difficult. Additionally, the impedance of the lowpass filter used in the double probe at the excitation frequency must beextremely large; for example, at least 100 kΩ when the excitationfrequency is 10 MHz, and the actual production thereof presentsdifficulties. Furthermore, among all plasma parameters, only theelectron energy distribution of electrons having a comparatively highkinetic energy can be measured using the double probe, and themeasurement of the plasma potential and density, which is most necessaryin practice, can not be undertaken.

As described above, the instrument for measuring plasma excited by ahigh frequency in accordance with the present invention comprises a wirefor electrically connecting a first electrode disposed within a plasmaand a terminal provided outside a vacuum chamber which serves to takeout signals, and a first insulator which is provided so as to cover atleast a portion of the surface of the wire; the absolute value of theimpedance at a given frequency between the first electrode and theground when looking into the terminal side from the first electrode is 5times or more the absolute value of the impedance at this givenfrequency between the first electrode and the plasma in a state in whichno direct current flows through the first electrode. Accordingly, thepotential of the probe electrode disposed in the high frequencydischarge plasma fluctuates in such a manner as to accurately follow thefluctuations over time of the potential in the high frequency dischargeplasma space, so that the difference between the potential of the probeelectrode disposed in the high frequency discharge plasma and the plasmapotential does not vary even at high frequencies, and it is possible tomeasure all parameters accurately and simply by using a measurementmethod identical to that of conventional single probes. Furthermore, itis of course the case that the measurement of direct current dischargeplasma can be conducted using such a measuring instrument.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view showing Embodiment 1.

FIG. 2 is a circuit diagram showing a high frequency equivalent circuitof Embodiment 1.

FIG. 3 is a graph showing the current voltage characteristics ofEmbodiment 1 and a conventional example.

FIG. 4 is a schematic cross sectional view showing a modified example ofEmbodiment 1.

FIG. 5 is a schematic cross sectional view showing another modifiedexample of Embodiment 1.

FIG. 6 is a schematic cross sectional view showing Embodiment 2.

FIG. 7 is a schematic cross sectional view showing Embodiment 3.

FIG. 8 is a circuit diagram showing an impedance equivalent circuit ofEmbodiment 3.

FIG. 9 is a graph showing the relationship between the floatingpotential and the susceptance in Embodiment 3.

FIG. 10 is a circuit diagram showing a high frequency equivalent circuitof Embodiment 3.

FIG. 11 is a schematic view showing Embodiment 4.

FIG. 12 is a graph showing the relationship between the probe current,the electron current, and the probe voltage in a conventional example.

FIG. 13 is a graph showing the results of the measurement of a plasmausing a conventional probe having a variable reactance.

FIG. 14 is a graph showing the results of the measurement of a plasmausing a conventional probe having a variable resistance.

FIG. 15 is a conceptual diagram showing a conventional single probe.

(Description of the References)

101 electrode, 102 insulating tube, 103 electrode, 104 capacitor, 105insulating tube, 106 coaxial cable, 107 insulating tube, 108 capacitor,109 insulator, 110 wire, 111 filter, 201 electrode, 202 electrode, 203filter, 204 wire, 401 electrode, 402 insulating tube, 403 electrode, 404insulating film, 405 insulating tube, 406 coaxial cable, 501 electrode,502 insulating tube, 503 insulating tube, 504 coaxial cable, 601, 602coaxial cables, 603, 604 capacitors, 605 insulating tube, 606 insulatingtube, 607, 608 filters, 701 coaxial cable, 702 insulating tube, 703connector, 704 current input terminal, 705 connector, 706 variableimpedance circuit, 1101 electrode, 1102 resistor, 1103 coaxial cable,1104 insulating tube, 1105 insulating tube, 1106 connector, 1107 currentinput terminal, 1108 coaxial cable, 1109 high frequency voltagemeasurement apparatus, 1501 electrode, 1502 insulator, 1503 metal pipe.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the present invention will be explained in greater detailusing Embodiments; however, it is of course the case that the presentinvention is in no way limited to these Embodiments.

(Embodiment 1)

FIG. 1 shows a cross sectional view of a portion of an instrument formeasuring plasma which shows a first Embodiment of the presentinvention.

Reference 101 indicates a probe electrode which is disposed within theplasma; this is, for example, a tungsten wire having a diameter of 0.1mmφ. Reference 103 indicates a metal electrode which is also disposed inthe plasma; this is, for example, an aluminum or stainless steel pipehaving an outer diameter of 3.4 mmφ and inner diameter of 2.5 mmφ. Thisis connected with electrode 101 via capacitor 104, and is made floatingin a direct current manner. Capacitor 104 is a through-type capacitorof, for example, from 1,000 pF to 10,000 pF. The through-type capacitorcore electrode and electrode 101 are connected, for example, by means ofspot welding. If the capacity of the through-type capacitor isrepresented by C_(p), then the impedance of electrode 103 and the coreelectrode is given by 1/ωC_(p).

Insulating tube 102 is a ceramic tube having, for example, an outerdiameter of 1.4 mm and an inner diameter of 0.4 mm; it is affixed toelectrode 103 and capacitor 104 by means of, for example, an inorganicadhesive. If a comparatively narrow and long gap is provided betweeninsulating tube 102 and electrode 101, the conductive material comingfrom the plasma can not penetrate to the rear, so that it is possible toprevent the short circuiting of electrode 101 and electrode 103 by meansof attaching the conductive material.

Reference 106 indicates a coaxial cable; this is a semi-rigid coaxialcable having, for example, an outer diameter of the outer conductor of2.19 mm. One end of the core of the coaxial cable 106 is connected tothe core electrode of capacitor 104. The other end thereof is connectedwith the outer conductor of coaxial cable 106 via capacitor 108, and isfurther connected to wire 110. The length of coaxial cable 106 isapproximately equivalent to 1/4 the wavelength within coaxial cable 106at the excitation frequency of the plasma. The length may also be an oddmultiple thereof. In the case in which, for example, the excitationfrequency is 200 MHz, and the wavelength shortening coefficient of thecoaxial cable is 0.7, the length of coaxial cable 106 is 26.25 cm. Sincethis length is in proportion to the wavelength, it is possible that thecable will not extend into the chamber if the excitation frequency islow; however, in such cases, it presents absolutely no problem if theportion of the coaxial cable to the right of the break in FIG. 1, or theentirety of the coaxial cable, is disposed outside the chamber.

Capacitors 108 comprise, for example, 2000 pF chip capacitors; 4 ofthese are connected in parallel between the outer conductor of coaxialcable 106 and the core so as to form a total capacitance of 8000 pF. Itis of course the case that the number of capacitors 108 need not be 4.The absolute value of the impedance at 200 MHz with 8000 pF isapproximately 0.1 Ω; by means of these capacitors, the core and theouter conductor are essentially short circuited at high frequencies.

When this coaxial cable, which has a length equal to an odd multiple of1/4 of the wavelength, and one end of which is terminated with animpedance load which is sufficiently smaller than a characteristicimpedance, is looked into from the other end, the impedance appearsessentially infinitely large. That is to say, it functions as a filterhaving a large impedance at odd multiples of the excitation frequency.Reference 111 indicates a filter comprising the coaxial cable 106 andthe capacitors 108. Furthermore, the present probe possesses anadvantage which was not present in conventional probes, in that since acoaxial cable is employed in this portion, the probe can be freely bentwithin the chamber.

Reference 110 indicates a wire; this is, for example, an aluminum wirehaving a diameter of 0.5 mmφ, the surface of which is coated withalumina ceramics. Although not depicted in FIG. 1, one end of wire 110is connected, for example, to a current input terminal, and the currentwhich flows from the plasma into the electrode is conducted to theexterior of the chamber. Additionally, this is electrically connected toa constant voltage source via, for example, a low pass filter and adirect current ammeter. Of course, reference 110 need not necessarilyrepresent a wire; no problem is presented insofar as this referencerepresents a substance which can conduct current. Additionally, a lowpass filter, an ammeter, a constant voltage source, and the like may bedirectly connected to the core of coaxial cable 106, so that wire 110need not be present. Insulating tube 105 is a ceramic tube having, forexample, an outer diameter of 3.8 mm and an inner diameter of 2.2 mm; itis affixed to the coaxial cable 106 and electrode 103 using, forexample, an inorganic adhesive. This is provided in order to isolate theplasma, and to insulate the outer conductor of coaxial cable 106 fromelectrode 103. Insulating tubes 107 comprise, for example, ceramic tubesand are provided in order to electrically isolate the plasma from theouter conductor of coaxial cable 106. In this example, a number ofinsulating tubes having an inner diameter which is somewhat larger thanthe outer diameter of coaxial cable 106 are employed so that coaxialcable 106 is capable of bending. Insulator 109 comprises, for example,an inorganic filler, and is provided in order to electrically isolatethe plasma. It is of course the case that insulating tubes 107 andinsulator 109 need not be provided if no plasma is present in thevicinity thereof. Furthermore, insulating tubes 107 need not be providedeven if plasma is present in the vicinity thereof so long as contactbetween the coaxial cable 106 and plasma does not present a problem. 107and 109 may be coated with ceramics. At this time, since the coaxialcable can be comparatively freely bent, it is not necessary to providenotches therein.

FIG. 2 shows a high frequency equivalent circuit of the instrument forplasma measurement in accordance with Embodiment 1. References 201, 202,203, and 204 correspond to electrode 101, electrode 103, filter 111, andwire 110, respectively. Reference V_(p) indicates a plasma spacepotential, and Z_(s1), and Z_(s2) are impedances of sheaths executedaround electrodes 101 and 103. In actuality, a capacitor 104 is presentbetween electrode 101 and electrode 103; however, in comparison withZ_(s2), the impedance thereof is sufficiently small, so that it wasignored. Accordingly, 201 and 202 have the same potential at highfrequencies. One end of wire 110 is connected to a low pass filter whichis terminated by a capacitor, and is thus grounded at high frequencies.

In order to accurately measure all parameters of the high frequencydischarge plasma, it is necessary that the plasma space potential andthe potential of the probe electrode oscillate in a parallel manner, andthus it is necessary that the absolute value of the probe impedance besufficiently larger than the absolute value of the sheath impedance.

In the present probe, an electrode 103, which has a comparatively largesurface area in contact with the plasma, is provided in addition toelectrode 101, so that the sheath impedance is sufficiently smaller thanthat in conventional probes. The sheath impedance Z_(s) is expressed bythe following formula:

    Z.sub.s =Z.sub.s1 ×Z.sub.s2 /(Z.sub.s1 +Z.sub.s2)    (1)

Here, for example, Z_(s2) is 0.16 times Z_(s1), and Z_(s) is 0.14 timesZ_(s1). In contrast, in conventional probes, there is a only anelectrode corresponding to 101, so that the impedance of the sheath ison the level of Z_(s1). The difference between these two is clear.

A simple increase in the surface area of electrode 101 could beconsidered in order to reduce the sheath impedance; however, if this isdone, a large direct current flows into electrode 101, so that themeasurement itself disturbs the plasma. In the present probe, electrode103 is connected to electrode 101 via a capacitor, and a direct currentdoes not flow in electrode 103. The direct current flows only throughthe electrode 101, which has a small surface area, so that there isalmost no disruption of the plasma.

The present probe is characteristic not merely in that the sheathimpedance is small, but also in that the impedance of the probe at theexcitation frequency is large, as a result of the provision of filter111. For example, the absolute value of the impedance of filter 111 atan excitation frequency of 200 MHz is 6.2 kΩ. The reactance of wire 110is 160 Ω, and the absolute value of the impedance of electrode 101 withrespect to the ground is reliably more than 6 kΩ. In contrast, inconventional probes, this level was less than 200 Ω.

As a result of these two characteristics, the absolute value of, forexample, the impedance of the present probe is as much as 105 times theabsolute value of the impedance of the sheath, and highly accuratemeasurement of all parameters can be conducted. Next, an example thereofwill be discussed.

FIG. 3 shows the current/voltage characteristics of the direct currentof the probe. The horizontal axis indicates the voltage applied to theprobe from the exterior, the left hand vertical axis indicates thecurrent value of the probe, and the right hand axis indicates theelectron current value; solid lines A and B and dotted lines C and Dindicate, respectively, measured values of the probe of the presentembodiment and a conventional single probe, and A and C indicate theprobe current, while B and D indicate the electron current. The plasmaconditions in both cases were completely identical; however, the resultsobtained in the case of the conventional single probe showed a voltagewhich was shifted by as much as 30 V lower, so that it can be seen thatthe range in which the voltage of the electron current increases and therange of the portion which is in agreement with the straight line priorto saturation are narrow. It was learned that the more the plasmapotential and the potential of the probe electrode oscillate in aparallel manner, the greater the increase in the voltage of thecurrent/voltage characteristics, and the more possible an accuratemeasurement of all plasma parameters. Furthermore, the plasma potentialof the same plasma as determined by means of an analysis of ion energy,which is a completely separate measurement method, was 24 V, and thiswas in agreement with the results of the measurement by means of a probeof the present embodiment. From these facts, it was proved that themeasurement of plasma excited by a high frequency, which was completelyinaccurate when a conventional probe was used, could be accuratelycarried out by using the present probe.

Next, a modified example of the portion marked B in FIG. 1 will beexplained. The outer conductor of the coaxial cable 106 of FIG. 1 is ina floating state; however, no problems will be caused even if it isgrounded. In such cases, the end of coaxial cable 106 on the side ofwire 110 is grounded, and it is no longer necessary to take theimpedance of wire 110 into consideration.

In addition, it is permissible not to provide capacitor 108, and toshort circuit the core and outer conductor of coaxial cable 106 at theside at which capacitor 108 was applied. In this way, use is possible atstill higher frequencies, and the structure is also simplified. However,in this case, it is necessary to place the outer conductor in a floatingstate in a direct current manner. Accordingly, in the case in which theouter conductor is grounded, it is necessary to conduct this groundingvia a capacitor, and furthermore, it is necessary to completely isolatethe plasma so that no current flows from the plasma into the outerconductor.

Next, a modified example of the portion marked A in FIG. 1 is shown inFIG. 4. Reference 401 indicates an electrode which is disposed in aplasma and which corresponds to an electrode 101; this comprises, forexample, a tungsten wire having a diameter of 0.1 mm. Reference 403indicates an electrode which is disposed in the plasma, and comprises,for example, a circular copper plate having a diameter of 22 mm and athickness of 0.84 mm. The portions of this plate which come into contactwith the plasma are covered with an insulating film 404. Insulating film404 comprises, for example, alumina ceramics, and the thickness thereofis, for example, approximately 120 nm. 403 is continuous with 401.References 402, 405, and 406 are identical to, respectively, insulatingtube 102, insulating tube 105, and coaxial cable 106. Electrode 401 andelectrode 403 are connected, for example, by means of spot welding, andthe core of coaxial cable 406 and electrode 403 are connected by meansof, for example, soldering.

Electrode 403 is continuous with electrode 401; however, because thiselectrode is covered by insulating film 404, no direct current flowsbetween it and the plasma. However, it is possible for the insulatingfilm 404 to function as a capacitor, and for a high frequency current toflow. Accordingly, by means of providing electrode 403, it is possibleto reduce the impedance of the sheath formed around the electrode athigh frequencies. That is to say, electrode 403 has the same function aselectrode 103 in FIG. 1, and the high frequency equivalent circuit iscompletely identical to that in FIG. 2. The characteristic features ofthe probe shown in FIG. 4 are that, since no capacitor is used betweenelectrode 401 and electrode 403, the structure is simplified, and use athigher frequencies is possible.

Next, a second modified example of the portion marked A in FIG. 1 isshown in FIG. 5. Portions identical to those in FIG. 1 are omitted.References 502, 503, and 504 correspond to, respectively, insulatingtube 102, insulating tube 105, and coaxial cable 106. Insulating tube502 and insulating tube 503, as well as insulating tube 503 and coaxialcable 504, are affixed by means of an inorganic adhesive. Reference 501indicates an electrode which is disposed in a plasma; this comprises,for example, the core of coaxial cable 504. Of course, other conductivematerials may be connected to the core and used as the electrode.

In the probe shown in FIG. 5, an electrode corresponding to electrode103 in FIG. 1 is not present, and the impedance of the sheath isapproximately the same as that in the conventional probe. However, ifthe absolute value of the impedance of the electrode is 5 times or morethe absolute value of the impedance of the sheath, accurate measurementis possible. In the present embodiment, the absolute value of theimpedance of the electrode at an excitation frequency of 200 MHz isapproximately 15 times the absolute value of the impedance of thesheath. However, the margin is smaller in comparison with that in FIG.1, and this probe is vulnerable to changes in the characteristics of thefilter resulting from fluctuations in the excitation frequency, changesin temperature, or the like. Because the structure is extremely simple,production is facilitated.

(Embodiment 2)

Next, a modified example of Embodiment 1 is shown in FIG. 6. In thisembodiment, a new portion labeled B' is provided between the portionsmarked B and C. References 601 and 602 indicate coaxial cables; thesecomprise, for example, semi-rigid coaxial cables having a diameter of2.19 mm. The length of the external conductors of cables 601 and 602 isequivalent to, respectively, 1/4 and 1/8 the wavelength in the coaxialcable at the excitation frequency. The length of these cables may alsobe an odd multiple thereof. References 603 and 604 indicate capacitorswhich correspond to reference 108; these comprise, for example, chipcapacitors having superior high frequency characteristics. Reference 605corresponds to insulating tube 107. Reference 606 comprises aninsulating tube; this is, for example, a ceramic tube having an innerdiameter of 4 mm and an outer diameter of 5 mm. This is provided inorder to isolate the plasma and to connect coaxial cables 601 and 602.Reference 607 functions as a filter comprising coaxial cable 601 andcapacitor 603; reference 608 functions as a filter comprising coaxialcable 602 and capacitor 604. The difference between filters 607 and 608is that the frequency at which they function as filters differs. Asexplained above, filter 607 has a large impedance at the excitationfrequency and at frequencies which are odd multiples thereof. Incontrast, filter 608 has a large impedance at odd multiples of 2 timesthe excitation frequency; that is to say, at frequencies which are evenmultiples of the excitation frequency. Since the space potential of theplasma generally does not oscillate in a sine wave pattern, higherharmonic components are included therein. Therefore, in order to bringthe probe electrode into completely parallel oscillation with the plasmapotential, it is necessary to increase the impedance of the probeelectrode not merely with respect to the excitation frequency, but alsowith respect to the higher harmonic components. By connecting filters607 and 608 in series as shown in FIG. 6, it is possible to provide alarge impedance with respect to all higher harmonic frequencycomponents. When higher harmonic components are present in theoscillation of the plasma potential, the probe shown in FIG. 6 iscapable of more accurate measurement than the probe of Embodiment 1.

Next, a modified example of the present embodiment will be discussed. InFIG. 6, a reversal of the order in which filters 607 and 608 aredisposed will cause no problem. Furthermore, a plurality of filters 607or 608 may be connected in series in order to increase the size of theimpedance. There is of course no restriction in the order ofdisposition.

Furthermore, a plurality of filters in which the length of the outerconductor of the coaxial cable is equal to an odd multiple of 1/4 thewavelength in the coaxial cable at each excitation frequency, or an oddmultiple of 1/8 the wavelength, may be connected in series in order tomeasure a plasma in which, for example, a plurality of excitationfrequencies exists.

Additionally, insulating tube 606 may be replaced by some other part, solong as that part is capable of isolating the plasma. It is of coursethe case that if plasma is not present at this portion it is notnecessary to provide such a part. Furthermore, in FIG. 6, coaxial cables601 and 602 are shown disposed in a straight line; however, it is ofcourse the case that these coaxial cables may be bent, and theconnecting portion may be set to a freely selected angle. It is alsopossible to provide other wiring between the cores of the coaxial cables601 and 602 at the portion at which they are connected.

Aside from the part labeled A in FIG. 6, the portion to the right of anyfreely selected spot in the probe may disposed outside the chamber.Furthermore, the modified examples described under Embodiment 1 may alsobe employed in the present embodiment.

(Embodiment 3)

In FIG. 7, a probe is shown in which the portions labeled B and C aremodified. Coaxial cable 701 is, for example, a semi-rigid coaxial cablein which the diameter of the outer conductor is 2.19 mm, and the lengthof the outer conductor is 192 mm. A connector 703 is connected to theright end of coaxial cable 701. Reference 702 corresponds to insulatingtube 107. Current input terminal 704 has connectors attached to bothends thereof and is affixed to the chamber wall. Reference 706 indicatescircuitry which enables the impedance on the side of 706 when lookingfrom connector 705 to be varied over a wide range.

The inner circuitry of reference 706 is shown in FIG. 8. In FIG. 8,V_(rf) and V_(dc) indicate, respectively, the potential of the core ofcapacitor 705 and the current potential applied to the probe electrode.References C₁ and C₂ indicate variable capacitors having a maximumcapacity of, respectively, 40 pF and 50 pF, while references L₁ and L₂indicate inductors of, respectively, 42 nH and 24 nH. References C₃ andL₃ indicate, respectively, a capacitor of 2000 pF and an inductor of 1.4μH; these form a low pass filter. The impedance between V_(rf) and theground can be varied by varying the capacitance of C₂ or C₁. Forexample, when C₁ is fixed at 24 pF and C₂ is made variable, theimpedance at 100 MHz can be set in a continuous manner within a range offrom -15.5 Ω to 60.5 Ω. Various circuit structures are possibledepending on the target impedance range. What is shown in FIG. 8 is onlyone example, and it is of course the case that other circuits arepossible.

Next, the superior features of a probe into which the impedance variablecircuit shown by reference 706 is built will be discussed.

FIG. 9 shows an example of the relationship between the potential of theprobe electrode and the susceptance between the probe electrode and theground, when the probe is made floating in a direct current manner. Inthis case, there is little probe power loss at the excitation frequency,so that this loss may be ignored. In FIG. 9, the black circles and thesolid line indicate the actually measured values, while the dotted lineindicates the calculated values. The calculated values were obtainedusing the high frequency equivalent circuit shown in FIG. 10. Referencesv_(p) and v indicate, respectively, the plasma potential and the probeelectrode potential. Resistance R_(P) and capacitance C_(S) are insertedin series between the plasma and the probe electrode, an v_(p1)indicates the potential of the portion connected to the plasma viaR_(p). The impedance is given by r+jx. References v_(p), v_(p1), and vare expressed by the following formulas.

    v.sub.p =V.sub.p exp (j(wt+φ))+v.sub.p

    v.sub.p1 =V.sub.p1 exp (jwt)+v.sub.p 1                     (1)

    v=V exp (j(wt+Ψ))+v

Here, V_(p), V_(p1), and V represent the amplitude of, respectively,v_(p), v_(p1), and v, and v_(p) , v_(p) 1, and v represent the averagevalues, respectively, of v_(p), v_(p1), and v. References φ and Ψindicate the phase differences between, respectively, v_(p) and v_(p1),and v_(p1) and v.

When V_(p1) and V are represented using V_(p), then the followingresults: ##EQU1##

By means of the voltage applied to the sheath, the thickness of thesheath changes, and the capacitance C_(s) also changes in accordancewith this. In the case of an ion sheath the thickness of the sheath isproportional to the voltage applied to the sheath raised to a power ofn1, and in the case of an electron sheath, the thickness is proportionalto the voltage raised to a power of n2. n1 and n2 are constants obtainedby fitting from the actually measured values of FIG. 9. Using a constantd obtained from the floating voltage of the probe, the sheathcapacitance at this time, and the current/voltage characteristics of theprobe, C_(s) is represented by the following formulas.

When v <v_(p) 1, ##EQU2## When v >v_(p) 1, ##EQU3##

R_(p) and V_(p1) are obtained by fitting from the actually measuredvalues of FIG. 9, and v_(p) and v_(p) 1 stand for the actually recordedvalues of the plasma potential when the susceptance is 0. If the valueof v is established, then v_(p1) and v can be obtained from Formulas (1)and (6).

A conduction current i such as that shown below, which depends on thevalues of v_(p1) and v or v_(p) 1 and v , flows in the probe electrode.

The electron current i_(e) follows the high frequency fluctuations, andis given by the following formulas.

When v<v_(p1), ##EQU4## When v>v_(p1), ##EQU5## Here, A is a constantobtained from the current/voltage characteristics of the probe. The ioncurrent i_(i) does not follow the high frequency, but is rather inaccordance with the average voltage.

When v >v_(p) 1, ##EQU6## When v <v_(p) 1, ##EQU7## Here, L and Mrepresent constants which are determined from the current/voltagecharacteristics of the probe. The average value of the conductioncurrent can be obtained by taking the time integral of i_(e) +i_(i). Inthis way, the current/voltage characteristics of the probe in a highfrequency discharge plasma can be calculated, and it is possible toobtain the floating voltage. By altering the impedance of the probe andconducting calculations, the characteristics shown by the dotted line inFIG. 9 can be obtained.

Incidentally, it can be seen in FIG. 9 that the actually measured valuesand the calculated values are in extremely close agreement. This meansthat the model discussed above is appropriate, and that the values ofthe fitting parameters n1 and n2 are accurate. That is to say, using theprobe of the present example, by measuring the dependence on thesusceptance of the current/voltage characteristics of the probe, it ispossible to accurately obtain the values not merely of the plasmapotential, density, and electron temperature, but also of thoseparameters particular to high frequency discharge plasma, such as theamplitude of the plasma potential, the sheath impedance, and the like.

It is possible to employ the probe of the present embodiment in such amanner that the size of the impedance of the probe electrode isincreased as in Embodiment 1 and 2, in addition to the method ofemployment described above. Moreover, because a impedance variablecircuit is provided, the probe can be adapted for use no matter how theexcitation frequency changes. For example, in the present embodiment, byadjusting the variable capacitor in the impedance variable circuit, itis possible to maintain the absolute value of the impedance of the probeat all frequencies from 84 MHz to 158 MHz at a level of approximately 6kΩ or more, so that accurate measurement is possible within thisexcitation frequency range. When a single impedance variable circuit isprovided, the frequency range is restricted; however, if a pluralitythereof are prepared, then it is in principle possible to cover allfrequencies by simply adding impedance variable circuits in accordancewith the frequency range. Furthermore, the excitation frequency and theimpedance of the probe electrode at this frequency need not be known. Asis clear from FIG. 9, when the susceptance is 0, that is to say, whenthe impedance is infinitely large, the probe voltage attains a maximumvalue, so that the impedance may be adjusted so that the floatingvoltage attains a maximum value.

Next, a modified example of the present embodiment will be discussed. InFIG. 7, it is sufficient if the coaxial cable 701 and the impedancevariable circuit 706 are electrically connected; the portions 703-705may have a structure other than that shown in FIG. 7. Furthermore, themodified examples explained under Embodiment 1 may be employed in thepresent embodiment as well.

(Embodiment 4)

The probes and the conventional probes discussed to this point measurethe current/voltage characteristics and extract the values of the plasmaparameters from the results of this measurement. The probe of thepresent embodiment which is shown in FIG. 11 is epoch-making in that itis capable of directly measuring the oscillation waveform and amplitudeof the plasma potential.

Reference 1101 indicates an electrode which is disposed within theplasma. This is, for example, a stainless steel mesh having anessentially square shape in which one side measures 28 mm, and in whichthe wire gap is 0.4 mm and the wire diameter is 0.3 mm. This electrodemay, of course, have a freely selected shape, and a metal mesh is notnecessarily required. Reference 1102 indicates a resistor, which isconnected to electrode 1101 by means of, for example, spot welding andis connected to the core of coaxial cable 1103 by means of, for example,soldering. Reference 1102 is a metal plated resistor of, for example,450 Ω. Reference 1103 indicates a coaxial cable, one end of which isconnected to connector 1106; this is, for example, a semi-rigid coaxialcable in which the outer diameter of the outer conductor is 2.19 mm.Current input terminal 1107 has connectors attached to both endsthereof, and is itself affixed to the chamber wall. Reference 1109indicates an instrument which is capable of measuring high frequencyvoltages; this comprises, for example, an oscilloscope in which theinput impedance is 50 Ω. It is of course the case that no device isruled out so long as it is an instrument which is capable of measuringhigh frequency voltages, such as a sampling oscilloscope, a frequencyanalyzer, or the like. Measuring instrument 1109 and current inputterminal 1107 are connected to a coaxial cable 1108, which hasconnectors attached to both ends thereof. References 1104 and 1105indicate insulating tubes; these comprise, for example, ceramic tubes.These are provided in order to isolate the plasma; however, insulatingtubes 1105 need not be present.

In order to accurately measure the oscillation waveform of the plasmapotential, it is necessary that the potential of electrode 1101oscillate in such a manner as to be synchronous with and parallel to theplasma potential. For this reason, it is necessary to increase the sizeof the impedance between electrode 1101 and the ground, and in the probeof the present embodiment a resistor 1102 is provided. Furthermore, thesurface area of the electrode 1101 is made comparatively large in viewof the necessity of decreasing the sheath impedance.

The end of the core of the coaxial cable 1103 which is on the side ofresistor 1102 is the point labeled D in the diagram. Measuringinstrument 1109 measures a potential which is equivalent to thepotential at the D point. The potential at point D has a value equal tothe potential of electrode 1101 divided by resistance 1102 and theimpedance when looking into the side of the measuring instrument 1109from the D point. For example, when the resistance 1102 is 450 Ω, andthe impedance when looking into the side of the measuring instrument1109 from the D point is 50 Ω, then the measuring instrument measures apotential which is 1/10 the potential of electrode 1101.

Next, a modified example of a probe of the present embodiment will bediscussed. In FIG. 11, it is sufficient that coaxial cable 1101 andmeasuring instrument 1109 be electrically connected; the portion1106-1108 may have a structure other than that shown in FIG. 11.

Furthermore, when the impedance when looking into the side of themeasuring instrument 1109 from point D is capacitive, a capacitor may beemployed in place of the resistor 1102. When the length from point D tothe measuring instrument 1109 is sufficiently shorter than thewavelength at the excitation frequency, then when a high input impedancemeasuring instrument 1109 is employed, the impedance when looking intothe side of the measuring instrument 1109 from point D is comparativelylarge and capacitive. In such a case, a capacitor may be used in placeof resistor 1102, and the potential of electrode 1101 divided.Furthermore, when a structure is employed such as in the presentembodiment in which an oscilloscope is connected and the waveform isdetermined, then when the impedance when looking into the side of themeasuring instrument 1109 from point D is 3 or more times larger thanthe sheath impedance, point D may be directly connected to electrode1101.

Furthermore, an impedance variable circuit may be inserted betweenelectrode 1101 and resistor 1102. In an impedance variable circuit, theimpedance on the electrode 1101 side, that is to say, the input side, islarger than the impedance on the output side; this is, for example, avoltage follower circuit employing an operational amplifier. In theprobe shown in FIG. 11, if the resistance of the resistor 1102 is madetoo large, then the voltage measured by measuring instrument 1109 willbecome small, and furthermore, the parasitic capacitance of the resistoritself can no longer be ignored. For this reason, it is impossible tomake the impedance of the electrode extremely large. In contrast, in aprobe in which an impedance variable circuit is employed, it is possibleto increase the size of the electrode in accordance with the inputimpedance of the variable circuit. This is structurally complex;however, such a probe has advantages in that the accuracy is high, thefrequency band is wide, and electrode 1101 can be made smaller.

INDUSTRIAL APPLICABILITY

As described above, by means of the instrument for measuring plasma inaccordance with the present invention, the measurement of plasma excitedby a high frequency, which was conventionally almost impossible, can beconducted in an accurate and simple manner. That is to say, themeasurement of the average potential, the oscillation waveform of thepotential, the density of the plasma, and the like becomes possible forthe first time, and this is extremely effective in the analysis ofplasma excited by high frequencies. In particular, this is effective inprocess apparatuses used in the formation of various types of thinfilms, pattern etching, and the like, which employ plasma excited by ahigh frequency. The most important value in such plasma processes is theenergy of the ions irradiated onto the plasma surface which conduct filmformation or etching, and the control of this irradiated ion energy atthe optimal value in the various processes is a necessary condition foran increase in the quality of the process; by means of accuratelymeasuring the plasma potential, the irradiated ion energy can also beaccurately controlled.

We claim:
 1. A measuring instrument for measuring all values of a plasmaproduced by means of a high frequency discharge at a given frequencywithin a vacuum chamber, said measuring instrument comprising: a wirefor electrically connecting a first electrode disposed within a plasmaand a terminal disposed outside said vacuum chamber for providing outputsignals; and an insulator covering at least a portion of a surface ofsaid wire; wherein the absolute value of the impedance at said givenfrequency between said first electrode and ground via said wire is atleast five times the absolute value of the impedance at said givenfrequency between said first electrode and said plasma in a state inwhich no direct current flows through said first electrode.
 2. Ameasuring instrument for plasma excited by a high frequency inaccordance with claim 1, further comprising a second electrode disposedin said plasma wherein said second electrode and said first electrodeare connected via a capacitor.
 3. A measuring instrument for plasmaexcited by a high frequency in accordance with claim 1, furthercomprising circuitry coupled to said terminal such that the impedance atsaid given frequency between said terminal and ground via said circuitryis variable.
 4. A measuring instrument for plasma excited by a highfrequency in accordance with claim 1, wherein at least a portion of saidwire comprises a coaxial cable, a length of an outer conductor of saidcoaxial cable is approximately equivalent to an odd multiple ofone-fourth of a wavelength associated with said given frequency, and acore of said coaxial cable and said outer conductor of said coaxialcable are electrically connected via a capacitor at an end of saidcoaxial cable coupled to said terminal.
 5. A measuring instrument forplasma excited by a high frequency in accordance with claim 1, whereinat least a portion of said wire comprises a coaxial cable, a resistor isdisposed between said first electrode and a core of said coaxial cable,and a measuring device for measuring the high frequency voltage at saidterminal is coupled to said terminal.